The present invention relates to a frame format for PCM speech data in a telephone transmission system, and to a digital telephone apparatus for use with this frame format. More particularly, the invention relates to a digital telephone transmission system which utilizes a standard, four-wire, telephone transmission line and operates in the full duplex mode for communication between subscriber stations, attendant consoles and data recording, storage and processing equipment.
As is well known, the conventional telephone apparatus is an entirely "analog" device and comprises an acoustic/electrical transducer or microphone, an electrical/acoustic transducer or earphone, a hook switch, a dialing mechanism and a bell or buzzer. As the cost of electronic equipment is currently falling, many "features" may be added to this otherwise remarkable invention.
Some typical features which a telephone instrument may include are:
As these and other features and functions are added to the telephone instrument, it can become a most convenient and versatile piece of equipment. Not only will the telephone in the future serve each user as an instrument of voice and data communications, it will also serve other telecommunication functions such as providing music, external paging, message recording and/or dictation access; room and building security against unauthorized access and/or fire; and room and building energy control such as lighting, heat and air conditioning. As noted above, the telephone instrument may also serve as a small, handy computer which can operate off-line or in direct communication with another computer at another location.
When designing telephone keyset apparatus--that is, a telephone instrument with a number of keys for dialing and for other functional purposes--it is desirable to take into consideration and to provide for the possibility of implementing the various features and functions such as those enumerated above. One of the important requisites for a telephone apparatus of this type is the ability to connect to a standard, four-wire telephone transmission line and to operate with full duplex communication. Another important requisite is that the apparatus transmit and receive information in digital, not analog, form. Digital telephone systems known in the art utilize two different types of solutions to provide duplex communication with sufficient transmission quality. The first technique requires a plurality of transmission lines: that is, separate lines for PCM voice data in each direction, for signalling and for synchronizing. At least two of these wires are also used to provide DC power to the telephone apparatus. The second technique provides a so-called "ping-pong" type of duplex data transmission. With this technique, the transmission of data in opposite directions is alternately generated in the subscriber station/attendant console, one one hand, and in the telephone system or exchange, on the other. This data is transmitted in separate time periods via a two or four-wire transmission line.
Thus, whereas the first technique provides full duplex transmission by utilizing a multiplicity of wires, the second technique uses only a two or four-wire transmission line but sends data alternately back and forth between the telephone exchange and the subscriber station or attendant console.